Among rare earth permanent magnets, an R-T-B system rare earth permanent magnet has been adopted in various types of electric equipment for the reasons that its magnetic properties are excellent and that its main component Nd is abundant as a source and relatively inexpensive.
However, such an R-T-B system rare earth permanent magnet with excellent magnetic properties also has several technical problems to be achieved. A technical problem to be achieved is that since an R-T-B system rare earth permanent magnet has low thermostability, its coercive force is significantly decreased along with an increase in temperature. Patent Document 1 (Japanese Patent Publication No. 5-10806) proposes that heavy rare earth elements including Dy, Tb, and Ho as typical examples are added to enhance the coercive force at room temperature, so as to keep the coercive force to such an extent that it does not impair the use of the permanent magnet, even though the coercive force is decreased due to an increase in temperature.
An R-T-B system rare earth permanent magnet comprises a sintered body comprising at least main phase grains comprising R2T14B compounds and a grain boundary phase having a higher amount of R than the main phase. Patent Document 2 (Japanese Patent Application Laid-Open No. 7-122413) and Patent Document 3 (Japanese Patent Application Laid-Open No. 2000-188213) disclose the optimum concentration distribution of heavy rare earth elements in the main phase grains, which has a large influence upon magnetic properties, and a method for regulating such a concentration.
With regard to a rare earth permanent magnet, which comprises, as configuration phases, a main phase mainly comprising R2T14B grains where R represents one or more rare earth elements, and T represents one or more transition metals and an R rich phase where R represents one or more rare earth elements, Patent Document 2 proposes that heavy rare earth elements are distributed at a high concentration at least at 3 points in the above described R2T14B grains. Patent Document 2 describes that the R-T-B system rare earth permanent magnet is obtained by crushing each of an R-T-B system alloy comprising R2T14B as a configuration phase and an R-T system alloy wherein the area ratio of R-T eutectics containing at least one of heavy rare earth element is 50% or less, and then mixing them, followed by compacting and sintering. The R-T-B system alloy preferably comprises R2T14B grains as a configuration phase. It is recommended that the R-T-B system alloy have a composition consisting of 27 wt %≦R≦30 wt %, 1.0 wt %≦B≦1.2 wt %, and the balance being T.
Patent Document 3 discloses an R-T-B system rare earth permanent magnet, which comprises microstructures containing first R2T14B main phase grains having a concentration of heavy rare earth elements that is higher than that of a grain boundary phase and second R2T14B main phase grains having a concentration of heavy rare earth elements that is lower than that of a grain boundary phase, has a high residual magnetic flux density and a high value of the maximum energy product.
In order to obtain the aforementioned microstructures, Patent Document 3 adopts what is called the mixing method, which involves mixing two or more types of R-T-B system alloy powders containing different amounts of heavy rare earth elements such as Dy. In this case, regarding the composition of each type of R-T-B system alloy powders, the total amount of R elements is adjusted to be the same in all types of alloy powders. In the case of Nd+Dy for example, one type of alloy powders satisfies the composition of 29.0% Nd+1.0% Dy, and another type of alloy powders satisfies the composition of 15.0% Nd+15.0% Dy. In addition, regarding elements other than the R elements, it is preferable that all types of alloy powders contain substantially the same elements.
The R-T-B system rare earth permanent magnet described in Patent Document 2 has a coercive force (iHc) of approximately 14 kOe. Thus, it is desired that the coercive force be further improved.
Moreover, Patent Document 3 discloses a technique effective for improving the residual magnetic flux density and maximum energy product of an R-T-B system rare earth permanent magnet. However, it is difficult to obtain a sufficient coercive force by this technique. Thus, it is said that it is difficult to obtain both a high residual magnetic flux density and a high coercive force.
The present invention has been completed to solve the aforementioned technical problems. It is an object of the present invention to provide an R-T-B system rare earth permanent magnet capable of achieving both a high residual magnetic flux density and a high coercive force.